Effect of GnRH Dose on Occurrence of Short Oestrous Cycles and LH Response in Cyclic Dairy Heifers

Abstract

Prostaglandin F(2 alpha) (PGF(2 alpha)) and GnRH treatments given 24 h apart have been shown to result in short oestrous cycles (8-12 days) in some cows and heifers. The differences in responses may depend on the dose of GnRH. Therefore, the effect of the dose of GnRH on occurrence of short cycles and LH response was studied here. Oestrus was induced with dexcloprostenol (0.15 mg) in two groups of Ayrshire heifers. A second luteolysis was induced similarly on day 7 after ovulation; 24 h after PGF(2 alpha) treatment, the heifers were administered either a high (0.5 mg, n = 15, group T500) or low (0.1 mg, n = 10, group T100) dose of gonadorelin. Blood samples for progesterone analyses were collected daily from the second PGF(2 alpha) administration to the second ovulation after the PGF(2 alpha) injection. Beginning 24 h after the GnRH treatment, ovaries were examined by transrectal ultrasonography every 6 h until ovulation, and daily between day 4 and the next ovulation. Five heifers from both groups were sampled for LH analyses via a jugular catheter every 30 min from 1 h before to 6 h after the GnRH administration. Short oestrous cycles were detected in 7 of 10 cases in group T100 and in 12 of 15 cases in group T500. No significant differences in LH responses were detected between the groups. In group T500, the rise in LH concentration tended to be somewhat slower than in group T100. The dose of GnRH (0.1 vs 0.5 mg) did not affect the occurrence of short oestrous cycles and LH response.

Effect of progesterone on magnitude of the luteinizing hormone surge induced by two different doses of gonadotropin-releasing hormone in lactating dairy cows

“…The 48-h interval between GnRH injections in Chenault et al (1990) may have precluded a full pituitary response to GnRH in heifers previously receiving a desensitizing dose of some of the GnRH analogs. It is also intriguing that a lack of difference was observed in LH secretion in Holstein heifers receiving 100 compared with 500 μg of gonadorelin 24 h after a luteolytic dose of a synthetic PGF 2α analog (Rantala et al, 2009). Conversely, in agreement with our results, Dias et al (2010) reported that beef heifers treated with 200 μg of gonadorelin when mean P4 concentration was 6 to 7 ng/mL had greater mean LH concentration at 1 and 2 h after treatment than heifers receiving 100 μg of GnRH.…”

“…A potential strategy to enhance GnRH-induced LH release in lactating dairy cows may be to increase the dose of GnRH administered. Studies in cattle have reported diverse results using different doses of GnRH with either enhanced or similar LH release in response to increasing doses of GnRH (Chenault et al, 1990;Yamada et al, 2002;Colazo et al, 2009;Rantala et al, 2009;Souza et al, 2009;Dias et al, 2010). Much of the variation in these results may be attributed to factors associated with the experimental design of these studies, including the type and physiological conditions of the animals used, variations among GnRH products, GnRH doses administered, LH assay methods, parameters used to compare LH secretion, and the steroid milieu at the time of GnRH treatment.…”

Intravaginal instillation of gonadotropin-releasing hormone analogues with an absorption enhancer induced a surge of luteinizing hormone in lactating dairy cows

“…As expected, cows that had a surge of LH during the sampling period tended to ovulate earlier, and timing of ovulation for cows that ovulated in the G100-IM, G1000CA, and B80CA was within the expected time frame after GnRH treatment (Pursley et al, 1995;Rantala et al, 2009;Giordano et al, 2012a). In spite of the fact that cows in the G1000CA and B80CA treatments had a longer interval until LH concentrations reached their peak, mean time to ovulation was the same in all treatments.…”